Recently, a light-emitting element that provides a high luminance by applying a current to an organic compound, such as a light-emitting element using a luminescent organic compound (referred to as an organic light-emitting diode [OLED] or an organic EL element, and hereinafter referred to as an organic EL element), has been getting a lot of attention.
In the fundamental structure of an organic EL element, a layer including a luminescent organic compound (light-emitting layer) is sandwiched between a pair of electrodes. By applying a voltage to this element, electrons and holes are transported respectively from the pair of electrode to the light-emitting element, and a current then flows. Then, those carriers (electrons and holes) are recombined to generate an excited state by the luminescent organic compound, and luminescence is produced when the excited state returns to the ground state.
It is to be noted that the excited state generated by an organic compound can be types of a singlet excited state and a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
Since this organic EL element is usually formed by using an approximately submicron thin film, it is a great advantage that the organic EL element can be manufactured to be thin and lightweight. In addition, since the time from carrier injection to luminescence is on the order microseconds or less, it is also one of the features that the speed of response is quite fast. Further, since sufficient luminescence can be obtained at a direct voltage of several to several tens of volts, the power consumption is relatively low. From these advantages, organic EL elements have been getting a lot of attention as next-generation flat panel displays.
In addition, since the organic EL element is formed to have the shape of a film, planar light emission can be obtained easily by forming a large area element. This is a feature that is hard to obtain in point sources typified by a filament lamp and an LED or linear sources typified by a fluorescent light. Therefore, the organic EL elements also serve many uses as surface light sources that can be applied to lighting and the like.
However, these organic EL elements have problems with heat resistance and durability, which are drags on development of the organic EL elements. Since an organic EL element is usually formed by stacking organic thin films using organic compounds as typified in the following Patent Document 1, low durability of the organic compounds and the fragile organic thin film are considered to be causes of the problems described above.
On the other hand, there have been attempts to form a light-emitting element by using not an organic thin film but a layer in which an organic compound and an inorganic compound are mixed. For example, the following Patent Document 1 discloses a light-emitting element using a light-emitting layer in which fluorescent organic molecules are dispersed in a metal oxide. In addition, the following Patent Document 2 discloses a light-emitting element formed by stacking a layer in which organic compounds (a hole transporting compound, an electron transporting compound, and a luminescent compound) are dispersed in a silica matrix while being covalently bound to the silica matrix. In these references, it is reported that the durability and heat resistance of the element is improved.
(Non-Patent Reference 1) C. W. Tang, et al., Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987)
(Patent Reference 1) Japanese Patent Application Laid-Open No. 2-288092
(Patent Reference 2) Japanese Patent Application Laid-Open No. 2000-306669